What can happen is, minerals in the water can get inside the water valve and keep it from sealing. Also, make sure you release the foot pedal abruptly. This allows the spring to get the valve turned off completely. DO NOT slowly raise the pedal with your foot. Some people do this because they are embarrassed about the thump sound when releasing the toilet pedal. Doug
From Thetford FAQ section.
Water constantly runs or drips into my toilet's bowl. What should I do?
Within about 10 seconds after flushing the bowl will normally fill from 1-2 inches to provide a “water barrier.”
If the bowl continues to fill with water, the water valve is leaking and should be replaced.